To clarify the incidence of leukoencephalopathy in patients with t(1;19) and their clinical characteristics, we studied 239 acute lymphoblastic leukemia (ALL) cases. The 1;19 translocation was found in 20 (8.5%) of the 239 children with ALL. Leukoencephalopathy occurred in 2 (10%) patients with t(1;19) during the early first remission and in one case with t(1;19) at the time of central nervous system (CNS) relapse. Leukoencephalopathy was not found during the early first remission in patients lacking t(1;19), but did develop in 4 patients lacking t(1;19) at the time of CNS relapse. There were no differences in age, sex, leukocyte count, platelet count or serum lactate dehydrogenase level between t(1;19) patients with and without leukoencephalopathy. Our results suggest the incidence of leukoencephalopathy in patients with t(1;19) during the early first remission to be 10%, but we can not predict which patients will develop leukoencephalopathy.